During oil and gas drilling, a wellbore is drilled into a formation and then one or more strings of tubing or casing are inserted into the wellbore. For example, surface casing may line an upper portion of the wellbore and protrude out the top of the wellbore; one or both of production tubing and casing may be inserted into the wellbore to facilitate production; and intermediate casing, which is located between the production and surface casings, may also be present in the wellbore.
Gas migration and casing vent flow are both typical problems encountered during oil and gas drilling. For example, gas migration and casing vent flow can refer to any one or more of the following phenomena:                fluid flowing from the formation into an outermost annular portion of the wellbore behind an outermost casing string in the wellbore;        fluid flowing from the outermost annular portion of the wellbore into the formation; and        fluid flowing across any of the casing or tubing strings in the wellbore.In gas migration and casing vent flow, the moving fluid may be liquid or gaseous, and may eventually leak out of the wellbore and into the atmosphere, which harms the environment. Accordingly, when evidence of gas migration or casing vent flow is found, the location at which the fluid is flowing into the wellbore, the formation, or across the casing or tubing string is identified and a repair is performed. Such a process can be time intensive, costly, and inefficient.        
Accordingly, research and development continue into methods, systems, and techniques that can be used to more robustly and efficiently identify and repair occurrences of gas migration and casing vent flow.